Thermoplastic polyurethane polymers are linear non-crosslinked polymers produced by coreaction of difunctional reactants comprising hydroxyl terminated polyesters or polyethers, glycols, and diisocyanates. Thermoplastic polyurethanes vary in hardness depending on the ratio of glycol (hard) to polyester or polyether (soft) and produce soft thermoplastics known as elastomers or harder thermoplastics known as plastics Although high hardness and rigidity are desirable properties for molded plastic products, such products tend to exhibit poor low temperature properties such as poor resiliency, brittleness, low impact resistance and toughness, especially at temperatures about minus 30.degree. C. below room temperature for outdoor applications.
Polyethylene and polypropylene polymers are non-polar polymers while polyurethane polymers are polar polymers. Prior to this invention, it has been a generally accepted fact that thermoplastic polyolefin polymers are incompatible with thermoplastic polyurethane poly mers. Only additive amounts of polyolefins consisting of less than 3 percent by weight of the polymeric blend could be utilized.
For instance, U.S. Pat. No. 3,929,928 indicates that mill blending of a thermoplastic polyurethane with polyethylene results in severe plate-out due to incompatibility of the two polymers Researchers have reported in Organic Coatings Plastics Chemistry, Vol. 40, page 664 (1979), that it was impossible to prepare with a roll mill useful test specimens at any polyurethane/polyethylene blend ratios. Similarly, Walker's Handbook of Thermoplastic Elastomers, Section 5.4.17, reports that low density polyolefin modifications of polyurethane polymers must be maintained below 3 percent to avoid adverse effects due to incompatibility of the two polymers Althouh U.S. Pat. No. 3,272,890 purports useful blends of polyolefin and soft polyurethane polymers, such blends are polyolefin based containing less than 25 percent by weight polyurethane polymer where polyurethane polymer content above 25 percent are incompatible and cannot be molded into useful plastic containers. Crystalline high density polyethylene or polypropylene polymeric blends are even more difficult to prepare due to incompatibility of the crystalline polyolefins with polyurethanes. Useful blends of thermoplastic polyurethane elastomers containing less than 15 percent by weight neutralized ethylene/carboxylic acid copolymers are disclosed in U.S. Pat. No. 4,238,574 to provide elastomeric blends useful in blow-molding operations.
It now has been found that up to about 20 percent by weight of crystalline polyolefin polymer can be successfully blended with high hardness thermoplastic polyurethane elastomer by high shear mixing of melts of the respective polymers to produce a uniform, mechanically stabilized compatible polymeric blend of the two polymers. In accordance with the present invention, from about 3.5 to 20 weight parts crystalline polyolefin homopolymer or copolymer can be blended with 100 weight parts thermoplastic polyurethane polymer to produce a mechanically stabilized compatible polymeric mixture High shear blending of the polymeric blends can be effected by high shear mixers such as single or twin-screw extruders or a Buss-kneader at elevated temperatures above the melt temperatures of the respective polymers for time sufficient to produce the mechanically compatible polymeric mixtures. It has been found that conventional low shear mixers such as two-roll mills or a Banbury mixer provide inadequate shear and resulting macroscopically phase-separated incompatible polymeric blends.
The improved polyolefin modified polyurethane polymeric blends of this invention unexpectedly provide highly desirable compositions useful for molding plastic articles exhibiting considerably improved physical properties, especially low temperature impact properties, as well as improved hardness, resiliency, rigidity, and toughness. Still further advantages of the invention are obtained in the high shear processing step where improved processability due to reduced melt viscosity and reduced pressure fluctuation during processing are obtained by high shear processing of the polyolefin modified polyurethane blend. The polymeric blend is particularly useful for molding films, sheets, hoses, exterior automotive body parts, and similar plastic articles often subjected to low temperatures, for instance, about minus 30.degree. C. These and other advantages of the invention will become more apparent by referring to the detailed description of the invention and the illustrative examples.